There are several types of prior art dispensing systems used for dispensing metered amounts of liquid or paste for a variety of applications. One such application is the assembly of integrated circuit chips and other electronic components onto circuit board substrates. In this application, automated dispensing systems are used for dispensing dots of liquid epoxy or solder paste onto the circuit board substrates. The liquid epoxy and solder paste are used to mechanically and electrically connect components to a circuit board substrate. In such dispensing systems, it is typical that an alignment vision system is used to locate certain features on a substrate for the purpose of aligning the dispensing operations relative to certain features on the substrate. Exemplary dispensing systems are manufactured and distributed by Speedline Technologies, Inc. of Franklin, Mass.
It is also known in the field of automated dispensers to use the alignment vision system or another vision system to inspect the results of the dispense process. Typically, the optic axis of alignment or inspection vision systems are constructed and arranged to be substantially normal to the substrate. The direction normal to the substrate is typically referred to as a Z-axis. In conventional prior art cameras, this axis-normal configuration serves to maximize the focus across the field of view and to minimize the amount of distortion introduced. It is also typical that the dispenser in a dispensing system is constructed and arranged to be substantially normal to the substrate surface. This axis-normal configuration of the dispenser serves to maximize the quality and repeatability of the resultant dispensed material formation.
However, a consequence of having both the vision system and the dispenser arranged normal to the substrate as described is that they cannot both operate on the same substrate location at the same time. Accordingly, the center of the imaging system's field of view and the point of dispense are typically displaced laterally from each other.
Also typically included in conventional dispensing systems is a means for measuring the height of the substrate in the Z-axis. In some height measuring systems, physical contact is made between the measuring system and the surface to be measured. One such prior art height measuring system is described in U.S. Pat. No. 6,093,251 entitled “Apparatus for Measuring the Height of a Substrate in a Dispensing System”, which is incorporated herein by reference, and is assigned to the assignee of the present application. In other prior art height measuring systems, a laser light source and an optical sensing system are combined to measure the position of an object without making physical contact. Examples of such non-contact measuring systems include those manufactured and distributed by Micro-Epsilon of Ortenburg, Germany.
In such dispensing systems, whether the height measurement system incorporated is of the contact type or the non-contact type, it is typical that the point of height sensing is laterally displaced from the point of dispense.
As a consequence of the lateral offsets described above, movement times are associated with performing either height measurement or dispense inspection operations. As such, the throughput of the dispensing system is negatively impacted by frequent substrate height measurements or by frequent dispense inspection operations.
An additional consequence of the lateral offsets described above is the possible introduction of additional error mechanisms in the performance of certain types of calibration operations. For example, typically, precision positioning systems, such as those utilized in automated dispensing systems, are “mapped” as part of the manufacturing process. This mapping operation typically uses a vision system and a precision reference grid to learn and thus compensate for positioning errors in the positioning system. The precision reference grid may be of a chrome-on-glass type, well known to those skilled in the art, or an equivalent high-contrast grid on a suitable mechanically stable substrate. Examples of precision reference grids include those manufactured and distributed by Max Levy Autograph, Inc. of Philadelphia, Pa. The precision reference grid is placed in the machine under the view of the vision system, and the precision positioning system is commanded to move to a multitude of locations. At each location, the precision reference grid is viewed via the vision system, and the resultant image appropriately analyzed to determine the actual position achieved. By comparing the commanded positions with the actual positions achieved, positioning errors in the precision positioning system may be determined and saved. The learned positioning errors may then be applied to subsequent commanded positions to compensate for the positioning errors. However, if there are lateral offsets between the point where mapping is performed and the point where dispensing operations occur, and if any roll, pitch or yaw errors exist in the precision positioning system, then residual errors, commonly known as Abbe errors, may still remain unmeasured and thus uncompensated for.